I think the issue with MOND is that for each galaxy, you require several tuning parameters... which don't really have a logical or physical meaning.
Like, oh you need gravity to be just higher/lower in x regions, because... well just because.

I am sure there is more too it than that, but every MOND paper iv ever read struck me as that being the case, Same for conference talks.

Dark matter however, you have roughly two parameters, mass and shape of the halo. sure you do get some kind of volumetric density so it can get
complicated, but the simplest dark matter model of a roughly uniform blob of extra mass in a halo thoughout a galaxy, can be made to fit most rotation
curved to the zeroth order very well.

Still, im no expert on MOND.

For a galaxy to be a stable system... or semi-stable over many billions of years, having a rough gravity distribution, extra attraction because of
some 'MOND correction' I just cant help think it would give very specific shape artifacts in galaxies. Such as, star formation regions being ring like
rather than arm like...

I presume the black hole would have to be many more percentages more massive for there to not be needed dark matter theory, but even still, if the
black hole was made incrementally more massive for sake of these theoretical checks and balances, is it thought that the spiral rotations and outer
stars would 'make sense'?

I have to correct an assumption. Spiral rotations.

Stars in galaxies do not Spiral inwards, nor- are spiral arms a flow of stars/gas in towards the centre. They are simply regions of a galaxy that have
high star formation. High star formation regions will tend to produce an abundance of hot blue stars that live fast and die young, when they explode,
the shock of the material hitting the interstellar medium, promotes more star formation. While unobserved, the rotation of the stars is actually
slower than the 'apparent' rotation of the spiral. This is because the spiral is nt actually a rotation, but a shock front formation.

Ok well then the intrigue of my question is what causes the totality of stars and planets out of the apparent reach of the central black hole (stars
at the edge of milky way, spiral arms) from staying in the system of galaxy;

And the answer is dark matter, potentially? But how would significantly more mass near the outer edge of galaxy result in this circular like spiral
like rotation?

How would an analogy of the galaxy to the solar system be; is the way the solar system moves similar to how and why mechanically the galaxy operates
as a unit? Is it a smaller relative model?

A central mass which as a single object causes the majority of the gravity force, which causes all bodies surrounding as a system to move as a
system?

It is being said that the central black hole of milky way does not move the entire galaxy, that there is some other aspect of the galaxy, dark matter
maybe, maybe dark energy, which results in the detected galaxy system movement;

I think I get what you mean about shock front; that it is not like all stars and planets of milky way are spinning around the common central black
hole like a merry go round... but if that is true, is the milky way moving at all... or this is where we get into dark energy and inflation and you
say its not really moving, space is expanding, or something.

I presume the black hole would have to be many more percentages more massive for there to not be needed dark matter theory, but even still, if the
black hole was made incrementally more massive for sake of these theoretical checks and balances, is it thought that the spiral rotations and outer
stars would 'make sense'?

I have to correct an assumption. Spiral rotations.

Stars in galaxies do not Spiral inwards, nor- are spiral arms a flow of stars/gas in towards the centre. They are simply regions of a galaxy that have
high star formation. High star formation regions will tend to produce an abundance of hot blue stars that live fast and die young, when they explode,
the shock of the material hitting the interstellar medium, promotes more star formation. While unobserved, the rotation of the stars is actually
slower than the 'apparent' rotation of the spiral. This is because the spiral is nt actually a rotation, but a shock front formation.

Ok well then the intrigue of my question is what causes the totality of stars and planets out of the apparent reach of the central black hole (stars
at the edge of milky way, spiral arms) from staying in the system of galaxy;

And the answer is dark matter, potentially? But how would significantly more mass near the outer edge of galaxy result in this circular like spiral
like rotation?

How would an analogy of the galaxy to the solar system be; is the way the solar system moves similar to how and why mechanically the galaxy operates
as a unit? Is it a smaller relative model?

A central mass which as a single object causes the majority of the gravity force, which causes all bodies surrounding as a system to move as a
system?

It is being said that the central black hole of milky way does not move the entire galaxy, that there is some other aspect of the galaxy, dark matter
maybe, maybe dark energy, which results in the detected galaxy system movement;

I think I get what you mean about shock front; that it is not like all stars and planets of milky way are spinning around the common central black
hole like a merry go round... but if that is true, is the milky way moving at all... or this is where we get into dark energy and inflation and you
say its not really moving, space is expanding, or something.

During galaxy formation some energy is turned into rotation. There are stages, if it starts with a large cloud of gas like hydrogen then as
contraction starts some potential energy is liberated into kinetic energy - the motion of gas fragments. Then eventually individual galaxies form
around dark matter and so on.
The rotational forces help to flatten out the galaxy along the axis of rotation, so it's not just gravity here.

It's been a while since I read that stuff, so this is vague, look up galaxy formation or protogalaxy formation.

The reason the extra mass was needed can be explained by a cup of coffee. Say I take a spoon spin my coffee around and add milk in the center. It will
spin out from the center creating spiral arms. But say I do it again but this time drop the milk on the edge you will see it create a ring in the
outside if the cup with nothing in the center. A galaxy has to have enough mass outside the center to prevent the galaxy from flinging itself out into
space just like our coffee had to have enough mass around our milk to prevent it from all accumulating on the edge of the cup.

I think I get what you mean about shock front; that it is not like all stars and planets of milky way are spinning around the common central black
hole like a merry go round... but if that is true, is the milky way moving at all... or this is where we get into dark energy and inflation and you
say its not really moving, space is expanding, or something.

No - everything is in motion, the stars do orbit, but the point is that the Spiral arms being bright is partially just formation of blue stars, which
are inherently brighter, these die fast, (few million years for example) and the supernova resulting seeds the local area, and produces density waves
which promote star formation. so you kind of expect that as they live and die they produce high density regions that don't fully orbit with the stars
themselves... its more of like if you where to have LEDs on a disk that rotate slowly, but you make the LEDs flash in sequence so it makes the disk
look like it is rotating faster.

NOW

that isn't what dark matter is doing, as the measurements are of red and blue shift, NOT of stars turning off and on,

this is why the concept or conceptualization of what spiral arms are a bit difficult and what drives misconceptions

I know you've probably been asked this before so if you can give me the link, that would be great.

The concept of "nothing" and vacuum energy - isn't it true that this concept doesn't exist? Isn't vacuum energy fluctuations and transitions of
energy - particles form, are destroyed, more formed,etc. - maybe even dark energy? So the concept of absolute "nothing" is actually false? Links to
relevant articles would be appreciated. Thanks

a reply to: Phantom423
I think one of the best explanations I've seen for the layperson is this one, which discusses the unknowns of the vacuum from both the perspective or
relativity and from the perspective of quantum mechanics:

What Is Dark Energy?
That talks about dark matter too, but the part under the heading "What Is Dark Energy?" contains information relevant to your question.

In short, I think we have more questions than answers about the vacuum which is not well-understood. The source cites the "vacuum catastrophe" as
evidence of that though they don't call it that, they just say "the answer came out wrong - wrong by a lot". Yes, a 1 with 120 zeroes after it is a
lot.

But this puts that issue in a little better perspective I think, though it gets slightly more technical than the first source, but I think it has good
insight:

The reason the extra mass was needed can be explained by a cup of coffee. Say I take a spoon spin my coffee around and add milk in the center. It will
spin out from the center creating spiral arms. But say I do it again but this time drop the milk on the edge you will see it create a ring in the
outside if the cup with nothing in the center. A galaxy has to have enough mass outside the center to prevent the galaxy from flinging itself out into
space just like our coffee had to have enough mass around our milk to prevent it from all accumulating on the edge of the cup.

Yes, I have understood that; I was wondering what percentage of stars and planets was strictly kept in rotation by the central black hole, and where
in reference to distance from the black hole does the dark matter start showing its strongest effect;

I have always proposed that if the mechanism of gravity was better understood then that would help clear up at least some of the need and mystery of
dark matter;

the other thing that may help clear it up is considering the material nature of the matter and energy fields outside of the milky way, locally just
outside and increasingly distant, and how the locally just outside the milky way interacts with the just inside the milky way, Inside in terms of
strongest gravity, closest to the strongest center of mass,

What that last sentence is focusing on is the milky way as a gravitational system compared to 'that blackness' in between galaxies;

How the black hole is altering that ultimate gravity mechanism, to what percentage in terms of distance, from its center of mass and from its event
horizon (I would wonder if event horizon has a relation to any gravitational body of some point of no return, escape velocity, so the event horizon of
black hole is similar to escape velocity of earth; considering something like the escape velocity from the center of mass, might give one a notion of
the mass of the object);

and then where the effect of the central black hole has on the shape, the relation between all masses of milky way spatially, begins to drift off, and
the dark matter begins to be the cause of its section of the total form;

And then just beyond that, beyond the edge of milky way, one side of the border of inside of this gravitational shape milky way, through a material,
energetic, gravitational, mystery area, to a border, on the other side of which is intergalactic space; on that path from inside the milky way, to
outside, what is that total gravitational shape like; and what mass is causing exactly what in that gravitational field, to cause the total shape of
milky way to be what it is; where is the dark matter, and what is gravitationally causing it to stay in shape, and thus cause the milky way to stay in
the shape it is detected to be in?

Perhaps if galaxies are moving at all; there is a material space between galaxies, similar to the material space that is the gravity field, higgs
field, charge field, Em field, quantum foam, what have you,

And so the edges of galaxies; stars, planets, and whatever the gravitational shape the milky way as an object is at that edge, interacts with that
package of seemingly material givens, in an all pervading intergalactic space, and this interaction causes the stars and planets at the edge of the
milky way to not be flung off into inter galactic space, but to be physically compelled toward the common shape, of milky way galaxy, perhaps this
recoil effect even has some effect on the black hole, and how many stars approach it, or how fast it rotates if it does or where it travels,
potentially.

And so the edges of galaxies; stars, planets, and whatever the gravitational shape the milky way as an object is at that edge, interacts with that
package of seemingly material givens, in an all pervading intergalactic space, and this interaction causes the stars and planets at the edge of the
milky way to not be flung off into inter galactic space, but to be physically compelled toward the common shape, of milky way galaxy, perhaps this
recoil effect even has some effect on the black hole, and how many stars approach it, or how fast it rotates if it does or where it travels,
potentially.

The idea now is that quasars are just younger galaxies where the center supermassive black hole is still feeding on stars and huge amounts of energy
is being expelled out of the galaxy.
But in older galaxies like our own the remaining stars have settled into stable orbits around the center black hole so it's not feeding on stars and
releasing huge amounts of energy.

Some stars orbit's will eventually degrade and fall in however. But in the galactic center gravity from the black hole and nearby stars will be the
main factor in determining stars orbits.

So does the milky way move as a unit, and is it rotating, and if so would it be rotating around the black hole?

No, the milky way doesn't move
as a "unit". I'd say that a wheel moves as a "unit". The milky way doesn't move or rotate like a wheel, though it's a little bit more wheel-like than
it should be if the luminous matter was all there is, which creates the dark matter mystery.

What is the largest gravitational source keeping the dark matter in orbit/galaxy, and geometrically how does that dark matter being there
around the edges, result in the milky ways shape?

The dark matter itself I suspect is the biggest gravitational source affecting the dark
matter, and I suspect it's self-interacting gravitationally. Just look at the numbers I posted in a previous answer to your question about what
percent of the milky way mass is in the black hole. Most of the mass is in the halo according to the previous paper I cited by Kafle et al. I don't
know how accurate this model of the halo is since the matter is dark and we can't see it, but I think the model that Kafle et al used was something
along these lines, which is not just "along the edges" as you put it.

You can see not only the halo, but also the disk and bulge components referenced in the Kafle et al paper, in addition to the halo.

However there's a problem. Local searches for dark matter haven't found any significant amount in our solar system, nor have they been able to
gravitationally verify dark matter in a reasonably large area around our solar system which should have included dark matter according to this (I
cited some similar research earlier in the thread):

The most accurate study so far of the motions of stars in the Milky Way has found no evidence for dark matter in a large volume around the Sun.

According to widely accepted theories, the solar neighborhood was expected to be filled with dark matter, a mysterious invisible substance that can
only be detected indirectly by the gravitational force it exerts. But a new study by a team of astronomers in Chile has found that these theories just
do not fit the observational facts. This may mean that attempts to directly detect dark matter particles on Earth are unlikely to be successful.

That shows an alternate illustration of the dark matter halo, at least where we thought it was supposed to be. So it would be great if MOND or some
alternate explanation worked, but the alternate explanations like MOND do an even worse job of fitting the data as far as I can tell, unless you do a
lot of "curve-fitting" for each individual galaxy as Eros said.

This Kafle et al paper seems to conflict to some degree with the one cited above though perhaps it's not a direct contradiction because they're using
different sources of data and perhaps different assumptions, but the authors claimed to have measured the mass of the halo so how can they do that if
there's no dark matter in the halo? I don't think they could.

I really don't have the answers about dark matter and apparently nobody does, unless you do and you're holding out on us until you meet with your
professor-mentor. So you can ask any question about physics here, but sometimes the answer is "I don't know", which is the answer that seems to fit
many questions regarding dark matter.

So it is thought that entire red area, is 'full' of matter? Or what is the dark matter density in that red area? So it would be as if there were
millions of planets at the outer edge of the solar system like around pluto, and made a sphere around the solar system? But what was earlier said
about spiral galaxies is not really the stars you see in the image spiraling like that spiral shape, but really they die and then new stars appear...
or I dont know what was meant by that.

And you say the dark matter interacts with itself, gravitationally; to answer my question about if dark matter is needed to keep matter in area, then
is there a similar problem keeping dark matter in its area, which is interesting to say I guess.

What you need to consider when thinking of a dark matter halo as presented is the following

1) Only material in the halo on the inside of the orbit adds to the force of gravity experienced by the object orbiting towards the centre of mass.
External forces equate to zero (you can try it, mathematically it works)

2) The halo is a sphere, its mass goes proportional to r^3

3) The disk, is... not a 2d object but, it is basically a thin cylinder. its mass for the most part outside of the centre of the galaxy goes with
r^2

This page has quite a nice breakdown of it and how the different objects are modelled. What you see is that classical mechanics... just doesn't work,
and something else has to be going on. Hence dark matter halos stipulated as a solution... Exciting news on that front Arbitrageur, our detector is
complete and we will be pushing to cool it down and fill with Liquid Argon in the next week or two

originally posted by: ImaFungi
So it is thought that entire red area, is 'full' of matter? Or what is the dark matter density in that red area? So it would be as if there were
millions of planets at the outer edge of the solar system like around pluto, and made a sphere around the solar system?

I cited the Kafle et
al paper explaining how they calculated the mass of the halo so I suggest reading their explanation, they give details on mass and size and if you
know simple geometry calculating the density is simple.

The dark matter is unlike planets so forget your solar system analogy with extra planets. I guess the halo is more like an invisible "gas cloud"
except we know it's not a gas cloud because photons from distant sources passing through a gas cloud would interact with the particles in the gas
cloud revealing something about the presence of the gas cloud, and we don't see that. Well we do see some gas clouds in some places, which is how we
know these gas cloud detections work, but we don't see enough such matter to account for the mass inferred by gravitational observations.

But what was earlier said about spiral galaxies is not really the stars you see in the image spiraling like that spiral shape, but really they
die and then new stars appear... or I dont know what was meant by that.

And you say the dark matter interacts with itself, gravitationally; to answer my question about if dark matter is needed to keep matter in area, then
is there a similar problem keeping dark matter in its area, which is interesting to say I guess.

Eros I thought your answer was fine, and as
you can see he didn't understand my answer either

Imafungi, all analogies are wrong in some way, and this one maybe more than most, so I don't know if it will help your understanding or confuse you,
but watch the first 30 seconds of this stadium wave or maybe you've seen one before. The wave is moving around the stadium but the people aren't
moving around the stadium, the people are just "wiggling" (The stand up and wave their arms then sit back down).

Stadium wave

It's more complicated than that in a galaxy's density wave but the idea is the density increases in certain areas sort of like those people standing
up. One reason the analogy is completely wrong is the spiral density wave in galaxies might be related to elliptical orbits (refer to the schematic
diagram I posted), and in planetary ring systems they can be related to gravitational influences of moons, but the idea in the galaxy density wave,
planetary ring density wave, and stadium wave, is that the wave doesn't move at the same speed as the objects in the wave.

To put it another way, our solar system has been in and out of the bright spiral arms in our galaxy numerous times over billions of years, not totally
unlike the people in the stadium who are in and out of the wave without even moving. The stadium analogy would be a little more accurate if the people
were doing laps around the stadium while they made the waves, but hopefully you get the idea that the motion of the wave and of the people in it are
two different things.

originally posted by: ErosA433
Exciting news on that front Arbitrageur, our detector is complete and we will be pushing to cool it down and fill with Liquid Argon in the next week
or two

exciting times.

That is exciting news, thanks for the update! By the way I finally took the visual on-line tour of that facility you
work at, I was impressed. I knew there were some areas with strict clean-room type conditions like your new detector needed, but I wasn't really
expecting all the tunnels and hallways though them to have floors that looked clean enough to eat off of, but I guess that makes sense, since if the
tunnels/hallways were dirty, that dirt might find its way into the clean areas.

Here's to hoping you find something...it would be nice to solve this mystery.

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